Plasticity of IL-9-secreting T cells T helper subsets, through the secretion of specific cytokines, regulate an array of inflammatory diseases. The differentiation of Th cells into specific cytokine-secreting subsets is highly dependent on the surrounding cytokine environment and the transcription factors expressed within each subset. The Th9 subset, that secretes IL-9 as the defining cytokine, is the latest Th cell population to be described. Our lab recently showed that the transcription factor PU.1 promoted the development of these cells. Both IL-9 and PU.1 expression in T cells are required for maximal infiltration of the lung in models of allergic inflammation. Although, Th9 cells and Th2 cells share obligate transcription factors such as STAT6 and GATA3 and require IL-4 for their development, there is evidence that Th9 cells are a unique Th cell subset. PU.1 represents a switch factor by repressing Th2 cytokines and inducing IL-9. Thus, Th2 cells can acquire IL-9- secreting potential. However, whether the IL-9-secreting Th9 phenotype is stable, or whether Th9 cells may acquire cytokine expression associated with other Th subsets is not known. In this proposal, we will explore Th9 plasticity, and track Th9 cells to assess phenotype switching in vivo. In our first Aim we will determine the stability and plasticity of Th9 cells that either haveor lack expression of PU.1 using in vitro culture systems. In the second Aim we will generate Il9 reporter and lineage-tracing mice and test the presence and fate of IL-9-expressing cells in models of allergic inflammation. Our overall goal for this application is to define the plasticity f Th9 cells in allergic inflammation, and the role of PU.1 in directing this phenotype. The information learned from these studies will provide a greater understanding of the role this subset plays in allergic inflammation, and how targeting this subset, or its functions, might be developed as therapy for allergic disease in humans.